Electrically conductive fuel cell contact materials

ABSTRACT

A multilayer contact approach for use in a planar solid oxide fuel cell stack includes at least 3 layers of an electrically conductive perovskite which has a coefficient of thermal expansion closely matching the fuel cell material. The perovskite material may comprise La 1-x E x  Co 0.6 Ni 0.4 O 3  where E is a alkaline earth metal and x is greater than or equal to zero. The middle layer is a stress relief layer which may fracture during thermal cycling to relieve stress, but remains conductive and prevents mechanical damage of more critical interfaces.

BACKGROUND OF INVENTION

The present invention relates to a multilayer design of contact materials which may include electrically conductive perovskites.

High temperature fuel cells like solid oxide fuel cells comprise an electrolyte sandwiched between a cathode and an anode. Oxygen combines with electrons at the cathode to form oxygen ions, which are conducted through the ion-conducting ceramic electrolyte to the anode. At the anode, oxygen ions combine with hydrogen and carbon monoxide to form water and carbon dioxide thereby liberating electrons.

The fuel cells are stacked and interleaved with interconnect plates which distribute gases to the electrode surfaces and which act as current collectors. Contact pastes are used to bond the electrode to an interconnect and must therefore be electrically conductive. In U.S. Pat. No. 6,420,064, a cathode contact layer comprised of lanthanum cobaltate is disclosed.

Lanthanum cobaltate (“LC”), also known as lanthanum cobaltite, is a perovskite, which is a well-known class of mineral oxides characterized by a cubic or orthorhombic crystalline structure. Perovskites may be described by the formula ABO₃, where A represents divalent and/or trivalent ions and B represents trivalent and/or tetravalent ions, while the O atom is the oxygen ion. The divalent, trivalent and tetravalent ions may include La³⁺, Sm³⁺, Sr²⁺, Ca²⁺, Co³⁺, Ni³⁺, Fe³⁺, Cr³⁺, Mn³⁺ or Mn⁴⁺ amongst others. In cubic perovskites, this ABO₃ structure in a general sense can be thought of as face centered cubic (FCC) lattice with A atoms at the corners and the O atoms on the faces. The B atom is located at the center of the lattice.

Some perovskites such as LC are reasonably good electrical conductors. However, as a contact paste in a Ni—YSZ anode-supported SOFCs, LC suffers from one significant disadvantage. If sintered, its coefficient of thermal expansion is significantly greater than that of the bulk cell. Consequently, thermal cycling of the fuel cell results in large thermal stresses and the contact paste may break away from the cell and interconnect resulting in poor electrical contact.

In some cases, contact paste materials which display better interface performance with the cell can have poor interface performance with the interconnect.

Therefore, there is a need in the art for fuel cells having an improved contact paste with a multilayer design which is electrically conductive and which mitigates the difficulties in the prior art.

SUMMARY OF INVENTION

The present invention provides for a contact material comprising stress relief means for use in fuel cell stack between a fuel cell electrode and an interconnect. The contact material is electrically conductive and porous to permit the flow of reactant to the electrode. In one embodiment, the electrode is a cathode.

In one aspect, the invention comprises a fuel cell stack comprising a plurality of planar interleaved fuel cells and interconnects and comprising a contact layer disposed between at least one electrode of a fuel cell and an adjacent interconnect, the contact layer comprising at least two outer layers and a central layer of electrically conductive materials, wherein the central layer comprises, prior to a first thermal cycle, a stress relief layer comprised of: (a) fine or coarse particles of a conductive ceramic material combined with particles of a pore forming material; (b) a layer of a pore forming material; (c) two separately formed layers of coarse particles of a conductive ceramic material pressed together; or (d) two separately formed layers of coarse particles of a conductive ceramic wherein one or both layers comprise particles of a pore forming material.

If the stress relief layer comprises a layer of particles of a pore forming material, the stress relief layer may additionally comprise a separately formed layer of a conductive ceramic material which is pressed against the pore forming material. That separately formed layer may itself include particles of a pore forming material.

Upon operation, or upon heating to a suitable temperature, the pore forming material combusts, leaving a void space. If the pore forming material was introduced in particular form, the stress relief layer will be more porous and structurally weaker. If the pore forming material was introduced in a layer, a fracture plane will be left. Alternatively, a fracture plane may be introduced by forming two halves of a stress relief layer and contacting them without binding or otherwise linking them together.

Therefore, in another aspect, the invention may comprise a method of forming a stress relief layer in a contact layer disposed between at least one electrode of a fuel cell and an adjacent interconnect, the contact layer comprising at least two outer layers and a central layer of electrically conductive materials, wherein the central layer comprises the stress relief layer, said method comprising the steps of forming the stress relief layer from: (a) fine or coarse particles of a conductive ceramic material combined with particles of a pore forming material; (b) a layer of a pore forming material; (c) two separately formed layers of coarse particles of a conductive ceramic material pressed together; or (d) two separately formed layers of coarse particles of a conductive ceramic wherein one or both layers comprise particles of a pore forming material; and thermally cycling the stack to at least about 500° C.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of an exemplary embodiment with reference to the accompanying simplified, diagrammatic, not-to-scale drawing where:

FIG. 1 is a perspective view of an embodiment of a fuel cell unit of the present invention.

FIG. 2 is a cross-sectional view of an assembled fuel cell unit.

FIG. 3 is a SEM photograph of a multilayer contact material showing a fractured stress relief layer.

DETAILED DESCRIPTION

The present invention provides for a perovskite contact material which may be used to interface between a solid oxide fuel cell electrode and an interconnect or a current collector. When describing the present invention, all terms not defined herein have their common art-recognized meanings. The following description is of a single embodiment and certain variations. It is not intended to be limiting of the invention as defined in the claims.

A portion of a fuel cell stack is illustrated as an exploded view in FIG. 1 and in cross-section in FIG. 2. A single fuel cell (10) consists of an anode (12) supported structure having a thin electrolyte (14) and cathode (16) layer. A single fuel cell unit also includes an interconnect (18) which may be a monolithic plate having flow-directing ribs (20) stamped as shown in FIG. 1. The ribs (20) assist in providing an even distribution of air flow across the entire surface of the cathode between the air intake and exhaust manifolds. The cathode may comprise a composite material comprising a noble metal such as palladium and a ceramic such as yttrium stabilized zirconium, as described in co-owned U.S. Pat. No. 6,420,064, the contents of which are incorporated herein by reference.

The contact material (22) of the present invention is applied to one or both of the cathode and interconnect faces upon assembly of the fuel cell stack. The contact material may be applied by screen printing, as is well known in the art. In one embodiment, a layer may be screen printed onto the cathode surface and allowed to dry. The contact material paste dries as a porous green ceramic layer and may then be sintered prior to assembly of fuel cell units in the stack. Alternatively, the material may not be sintered before stack assembly, in which case, the contact material is sintered during operation of the fuel cell. A second contact layer is then applied (the fracture layer) and dried. Finally, a thin layer of the contact material as a wet paste is screen printed onto the cell dried layer(s) or the interconnect surface and the interconnect is then contacted with the cathode surface. If the interconnect is corrugated or ribbed, the contact material may or may not fill in the void areas of the interconnect. The contact material must be porous to allow reactants to flow from the interconnect and reach the fuel cell electrode.

Perovskites of the present invention may be described by the general formula ABO₃, where A is a doped or undoped rare earth metal, lanthanide or mixed lanthanide, and B is a doped or undoped transition metal, where the perovskite has a coefficient of thermal expansion (CTE) which closely matches that of the fuel cell electrode or the interconnect. A CTE is considered to closely match another CTE if it is within about 5×10⁻⁶K⁻¹ of the other CTE. The coefficient of thermal expansion of a material may be determined empirically or by estimation using known and published values. Whether or not two materials have closely matched coefficients of expansion may also be determined experimentally by thermal cycling the two materials adhered to each other and observing the loss of adhesion. For example, a contact material of the present invention may be applied to an interconnect or to a fuel cell electrode, and the two materials thermally cycled within the operating temperature range of a solid oxide fuel cell. If no, or substantially none, loss of adhesion is observed, then a person skilled in the art may conclude that the CTE's of the two materials are likely to be closely matched.

The transition metal may comprise cobalt, nickel, iron, copper, zinc or lead. In one embodiment, B comprises cobalt doped with nickel as follows: Co_(1-y)Ni_(y) where 0.3≦y≦0.7. Preferably, y is about 0.4. Nickel is a preferred material because the inclusion of nickel in the B-site tends to lower the coefficient of thermal expansion. Further, the perovskite formed with nickel is highly electrically conductive but is not very reactive with other materials.

The A element is preferably lanthanum and may be doped with an alkaline earth metal such as strontium, barium or calcium to improve electrical conductivity. Therefore, A may comprise La_(1-x)E_(x) wherein E is an alkaline earth metal and 0.0≦x≦0.8. Lanthanum cobalt nickel oxide materials are referred to herein as “LCN”.

A particularly preferred LCN material is La_(1-x)E_(x) Co_(0.6)Ni_(0.4) where x is greater than or equal to zero and less than about 0.7. Preferably, x is less than 0.5. The A and B elements may be stoichiometric or non-stoichiometric. If non-stoichiometric, the A:B ratio may vary from about 0.9 to about 1.1. The perovskites of the present invention may be applied as a paste using well-known solvents and binders to either or both of the cathode and interconnect in a fuel cell unit and sintered prior to assembly of the fuel cell stack. Alternatively, the paste may be unsintered prior to assembly of the fuel cell stack and sintered in situ upon operation of the fuel cell stack. Stack operating temperatures may reach about 800° C. Sintering additives to lower the sintering temperature of the perovskite may be desirable or necessary. Suitable sintering additives or aids such as copper, silver, glass frit or tin are well-known in the art.

A contact material of the present invention may also be used in the interface between the anode surface and an interconnect and its use is not restricted to the cathode surface.

In one embodiment, as shown in FIG. 3, the contact paste material is applied in a multilayer configuration which may provide better resistance to thermal cycling degradation and long term degradation. In one embodiment, the contact paste is applied in three layers in which the outer contact layers (100, 102) adhere to the fuel cell electrode and interconnect respectively, and the central layer comprises a stress relief layer (104). In one embodiment, the outer contact layers comprise fine conductive particles while the stress relief layer comprises coarse conductive particles. The conductive particles in either or both the fine and coarse layers preferably comprise conductive perovskites, including those perovskites described herein, or perovskites having a K₂NiF₄-type structure (e.g. La₂Ni_(1-x)CO_(x)O₄) or any other electrically conducting ceramic powder compatible with the fuel cell electrolyte and electrode materials.

As used herein, the term “fine” particles comprise particles having diameters less than about 2 μm and preferably about 0.3 to about 1.1 μm. As used herein, “coarse” particles comprises particles that are at least one and a half times the particle diameter of the fine particles, and preferably greater than about twice the diameter of the fine particles. Preferably, coarse particles have diameters greater than about 1 μm and more preferably greater than about 1.5 μm.

The stress relief layer (104) may be formed of a conductive ceramic material, such as the perovskites described herein, which has similar chemistry and similar sintering characteristics to the fine outer layers, but which comprises coarse particles. Alternatively, the stress relief layer may be formed from a conductive ceramic material which has significantly different sintering characteristics than the fine layers. For example, the stress relief layer may be formed of lanthanum strontium manganite (LSM), which has a significantly higher sintering temperature than LC or LCN. In this case, the stress relief particle size may be fine or coarse. In this case, the stress relief layer would not sinter or sinter to the same extent as the other layers. Alternatively, the stress relief layer may be formed of a porous metallic material such as expanded metal, or a fine metal mesh.

The stress relief layer may be porous or highly porous. In one embodiment, the stress relief layer comprises coarse particles and has a porosity of between about 25% to about 70%. Preferably, the stress relief layer may be about 30% to about 50% porous. Porous metallic stress relief layers may be more porous, up to about 95%.

The fine particle layers (100, 102) may be thinner or thicker than the coarse central layer. Preferably, the fine particle layers are less than about 25 μm thick while the coarse central layer may be about 10 μm to about 50 μm thick. The combined thickness of the multilayer contact materials may be about 60 to 120 μm, depending on the stack design and seal thicknesses. The combined thickness should preferably not exceed 200 μm.

The layers may be applied by screen printing a paste and sintered prior to stack assembly or left unsintered as described above. Sintering aids may be included if necessary or desired. The necessity or desirability of a sintering aid may be determined empirically by one skilled in the art.

In one specific embodiment, a layer of fine lanthanum cobalt nickel oxide (LCN) particles, as described above, is applied to the fuel cell electrode surface by screen printing. The LCN particles have an average particle size of about 1.0 μm with about 50% of the particles falling in the range of about 0.5 μm to about 1.1 μm. This layer of fine LCN particles may be less than about 25 μm thick and may or may not be sintered. Subsequently, a layer of coarse LCN material, as described above, is applied by screen printing onto the first fine layer and allowed to dry. The coarse LCN particles have an average particle size of between about 2 to about 3 μm, with a majority of the particles falling in the range between about 1 μm to about 10 μm. The remaining fine layer of LCN is screen printed onto this layer on the cell or the interconnect just prior to assembly of the stack. In an alternative embodiment, LC may be used in place of LCN in any or all of the layers.

The multilayered approach may provide better long term stability by providing a sacrificial fracture layer which absorbs expansion mismatches during thermal cycling and long term operation. The interfaces between the fine layers and the fuel cell and interconnect respectively remain intact while physical stresses are absorbed by the central stress relief layer. As shown in FIG. 3, a scanning electron micrograph demonstrates such a fracture in an autopsied fuel cell. The inventors have found that electrical conductivity through the contact material is maintained while the layers are compressed in a stack despite such horizontal fractures in the stress relief layer.

In one embodiment, the fracturing of the stress relief layer, or the interface between the stress relief layer and a fine outer layer, may be facilitated by the addition of pore forming material or a fracture layer. This may be desirable or necessary if the cathode contact material is not pre-fired. A pore former may be added to the stress relief layer which will have the effect of weakening the stress relief layer internally, facilitating the formation of fractures. Pore formers may comprise organic particulate matter and may include graphite, corn starch or polystyrene, or other materials which combust at or about 500° C. The pore former burns out before reaching the operating temperature of the fuel cell, or during a pre-sintering process, introducing voids in the structure of the stress relief layer. The pore forming particles may preferably be about 1 μm to about 50 μm in size, and more preferably less than about 20 μm. The pore forming particles may be added to the coarse particle composition which is screen printed or otherwise applied to a fine particle layer. Sufficient pore forming material may be added to a level of about 20 to about 60% by volume. With the addition of the pore former, the stress relief layer may be about 30–70% porous, preferably greater than about 50% porous, after combustion of the pore forming material. With larger pore forming particle sizes, the void spaces would of course be larger, but fewer in number.

In another embodiment, a weakened area may be introduced to the contact material by adding a fracture layer between the stress relief layer and the fine particle outer layer, or within the stress relief layer itself. The fracture layer may be added, for example, by adding a thin layer of an organic substance, which may the same as the pore forming substances described above. Suitable fracture layers may therefore comprise graphite, corn starch or polystyrene. Again, prior to reaching the operating temperature of the cell, or during a pre-sintering process, the fracture layer will burn out, leaving a weakened fracture plane. The fracture layer may be applied by spraying a suspension of the particles or by screen printing. Preferably, the fracture layer may be about 1–20 μm thick.

In another embodiment, a pre-fractured stress relief layer may be provided. In this example, a fine layer and a coarse stress relief layer may be applied to each of the cathode and the interconnect and allowed to dry. The coarse layer may or may not include pore formers as described above. When assembled, the respective stress relief layers are pressed together. The boundary between the two coarse stress relief layers forms a fracture plane, creating what is in effect a single pre-fractured stress relief layer. The two coarse stress relief layers may include pore forming particles as described above. Alternatively, a pre-fractured stress layer may be formed by pressing together a fracture layer formed from a layer of a pore forming material as described above and a coarse layer.

As will be apparent to those skilled in the art, various modifications, adaptations and variations of the foregoing specific disclosure can be made without departing from the scope of the invention claimed herein. The various features and elements of the described invention may be combined in a manner different from the combinations described or claimed herein, without departing from the scope of the invention. 

1. A fuel cell stack comprising a plurality of planar interleaved fuel cells and interconnects and comprising a contact layer disposed between at least one electrode of a fuel cell and an adjacent interconnect, the contact layer comprising at least two outer layers and a central layer of electrically conductive materials, wherein the central layer comprises, prior to a first thermal cycle, a stress relief layer comprised of: (a) fine or coarse particles of a conductive ceramic material combined with particles of a pore forming material; (b) a layer of a pore forming material; (c) two separately formed layers of coarse particles of a conductive ceramic material pressed together; or (d) two separately formed layers of coarse particles of a conductive ceramic wherein one or both layers comprise particles of a pore forming material.
 2. The fuel cell stack of claim 1 wherein the stress-relief layer comprises coarse particles and the outer layers comprises fine particles.
 3. The fuel cell stack of claim 2 wherein the coarse particles have an average diameter at least about 1.5 times the average diameter of the fine particles.
 4. The fuel cell stack of claim 3 wherein the outer layers comprises particles having an average diameter of less than about 2 μm and the central layer comprises particles having a diameter of greater than about 1.5 μm.
 5. The fuel cell stack of claim 4 wherein the central layer comprises LCN particles.
 6. The fuel cell stack of claim 5 wherein the outer layers comprise LC particles.
 7. The fuel cell stack of claim 1 wherein the outer layers comprise fine LC or LCN particles and the stress relief layer comprises fine LSM particles, or coarse LSM particles, or coarse LCN particles.
 8. The fuel cell stack of claim 7 wherein a first outer layer contacting the electrode comprises fine LCN particles, a second outer layer contacting the interconnect comprises fine LC particles, and the stress relief layer comprises coarse LCN particles.
 9. The fuel cell stack of claim 1, wherein the stack has been thermally cycled above a temperature of about 500° C., such that the pore forming particles have combusted, resulting in void spaces.
 10. The fuel cell stack of claim 1 wherein the outer layers comprise fine LC or LCN particles and the stress relief layer comprises fine LSM particles, or coarse LSM particles, or coarse LCN particles with a pore forming material.
 11. The fuel cell stack of claim 10 wherein the stress relief layer comprises coarse LCN particles with a pore forming material in the range of 20–60% by volume.
 12. A method of forming a stress relief layer in a contact layer disposed between at least one electrode of a fuel cell and an adjacent interconnect, the contact layer comprising at least two outer layers and a central layer of electrically conductive materials, wherein the central layer comprises the stress relief layer, said method comprising the steps of forming the stress relief layer from: (a) fine or coarse particles of a conductive ceramic material combined with particles of a pore forming material; (b) a layer of a pore forming material; (c) two separately formed layers of coarse particles of a conductive ceramic material pressed together; or (d) two separately formed layers of coarse particles of a conductive ceramic wherein one or both layers comprise particles of a pore forming material; and thermally cycling the stack to at least about 500° C. 